The present invention relates to bonding materials for connecting members to be connected having opposite electrodes, particularly to bonding materials suitable for connecting members to be connected having electrodes undergoing high voltage and high current.
Anisotropic conductive films have been substituted for solders conventionally used as bonding materials for connecting members to be connected having opposite electrodes. These anisotropic conductive films are bonding materials comprising conductive particles dispersed in a thermosetting resin and they are inserted between members to be connected for thermocompression bonding so that the conductive particles come into contact with electrodes to establish electric connection between the electrodes while the resin is cured with the conductive particles dispersed therein to ensure insulation between adjacent electrodes and mechanical bonding between members to be connected in regions where electrodes are not present.
Such anisotropic conductive films have anisotropic conductivity allowing opposite electrodes to be electrically connected while adjacent electrodes are insulated. They are used to mount a semiconductor element on a printed wiring board or to connect a printed wiring board to another printed wiring board such as a flexible printed wiring board or to connect a printed wiring board to an ordinary liquid crystal display device.
However, these anisotropic conductive films are generally considered to be suitable for connecting electrodes at low voltage and low current but ineffective for ensuring electric connection or insulation when high voltage or high current is applied across electrodes. For example, it is difficult to connect a plasma display panel and a flexible printed wiring board via an anisotropic conductive film but solder is still used for this purpose because plasma display panels undergo high voltage and high current at 50 V through 500 V and 500 mA through 2 A.
Investigations into the causes of unsuitability of anisotropic conductive films for high voltage or high current applications revealed that a major cause is short circuit between adjacent electrodes due to migration of metal ions from electrodes under conduction. Such migration was found to increase with voltage or current.
An object of the present invention is to provide a bonding material with excellent adhesion, electric connection and insulation properties, which can be used to connect members to be connected having opposite electrodes for high voltage or high current applications without inducing short circuit between adjacent electrodes even under the application of high voltage or high current.
The present invention provides a bonding material for connecting members to be connected having opposite electrodes, the bonding material comprising conductive particles, metal ion scavenger particles for scavenging metal ions liberated from the electrodes, and a resin component based on a thermosetting resin, wherein the metal ion scavenger particles have an average particle diameter smaller than that of the conductive particles.
In the bonding material of the present invention, the metal ion scavenger particles comprise at least one of a bismuth-based ion exchanger and a vinyl triazine compound.
In the bonding material of the present invention, the metal ion scavenger particles are present 1.5 parts by weight or more but 60 parts by weight or less per 100 parts by weight of the resin component.
In the bonding material of the present invention, the metal ion scavenger particles are present 3 parts by weight or more but 50 parts by weight or less per 100 parts by weight of the resin component.
In the bonding material of the present invention, the conductive particles have an average particle diameter of 1 xcexcm or more but 60 xcexcm or less and the metal ion scavenger particles have an average particle diameter of 0.1 xcexcm or more but less than 10 xcexcm.
In the bonding material of the present invention, the metal ion scavenger particles have a specific surface area of 0.8 m2/g or more but 100 m2/g or less.
The present invention also provides a bonded assembly comprising opposite electrodes and a bonding material inserted between the electrodes and heat-cured to at least electrically connect the electrodes, the bonding material comprising conductive particles, metal ion scavenger particles for scavenging metal ions liberated from the electrodes, and a resin component based on a thermosetting resin, wherein the metal ion scavenger particles have an average particle diameter smaller than that of the conductive particles and wherein the voltage applied across the opposite electrodes is 50 V or more but 500 V or less.
Members to be connected according to the present invention are any members having opposite electrodes, particularly a number of electrodes, but the present invention is suitable for connecting members having electrodes undergoing a high voltage of 50 V or more but 500 V or less, especially 70 V or more but 300 V or less or a high current of 100 mA or more but 10 A or less, especially 200 mA or more but 5 A or less. Connections between such members include connection between a plasma display panel as described above or a printed wiring board for driving it and a flexible printed wiring board to be connected thereto.
These members to be connected have a substrate such as a glass or resin substrate, and flexible printed wiring boards often have a resin substrate made of a polyimide resin. Electrodes formed on these substrates may contain metals dissociating as metal ions such as silver, copper, nickel or chromium but may also contain non-dissociating metals.
Bonding materials of the present invention comprises a thermosetting resin, conductive particles and a metal ion scavenger, and they are inserted between members to be connected and pressurized from both sides so that opposite electrodes are forced into contact with the conductive particles while the resin is concentrated and cured with the conductive particles dispersed therein to bond the members in regions where electrodes are not present, whereby electric connection and mechanical bonding are achieved.
Base resins of thermosetting resins used in bonding materials of the present invention may be any resins that are cured under heating or irradiation such as UV in combination with curing agents, such as epoxy resins, urethane resins, phenol resins, hydroxyl-containing polyester resins or hydroxyl-containing acrylic resins, but preferably epoxy resins in view of the balance of the curing temperature, curing time, storage stability, etc.
Suitable epoxy resins include bisphenol-type epoxy resins, epoxy-novolac resins or epoxy compounds having two or more oxirane groups in their molecule or the like. These resins are commercial available.
The above base resins of thermosetting resins may be generally cured in combination with curing agents, but curing agents may be omitted when a functional group contributing to curing reaction is attached to the base resins. Suitable curing agents are those capable of inducing curing reaction with base resins under heating or irradiation, such as imidazole, amines, acid anhydrides, hydrazides, dicyandiamide and modifications thereof, and are also commercially available. These curing agents are preferably latent curing agents.
Latent curing agents induce curing reaction under heat and pressure (thermocompression bonding) or irradiation such as UV at the curing temperature but not during preparation and storage at normal temperatures and drying under relatively low-temperature conditions in the range of 40xc2x0 C. through 100xc2x0 C. Especially preferred such latent curing agents are the above curing agent components such as imidazole or amines microencapsulated and are also commercially available. Heat-activatable curing agents preferably begin curing at a temperature range of 80xc2x0 C. through 150xc2x0 C.
Suitable conductive particles include metal particles such as solders or nickel; conductive material-coated particles comprising core particles of a polymer organic compound coated with a conductive material by plating or the like; or insulating material-coated conductive particles comprising these conductive particles coated with an insulating resin. These conductive particles suitably have an average particle diameter of 1 m or more but 20 xcexcm or less, preferably 3 xcexcm or more but less than 10 xcexcm. As used herein, the average particle diameter means the average of the diameters of particles.
Metal ion scavengers are compounds preventing migration by scavenging metal ions dissociating from constituent materials of electrodes, and include ion exchangers, complexing agents or the like, either inorganic or organic. Inorganic species include bismuth-based ion exchangers, antimony-based ion exchangers, bismuth-antimony-based ion exchangers or the like, and organic species include vinyl triazine compounds or the like.
Bismuth-based ion exchangers are ion exchangers containing bismuth as a constituent such as BiO(OH), BiO(OH)0.7(NO3)0.3, BiO(OH)0.74(NO3)0.15(HsiO3)0.11 or the like. Antimony-based ion exchangers are ion exchangers containing antimony as a constituent such as Sb2O2.2H2O. Bismuth-antimony-based ion exchangers are ion exchangers containing bismuth and antimony as constituents such as compositions containing a bismuth-based ion exchanger and an antimony-based ion exchanger as defined above in any ratio such as 5:5 through 7:3.
These ion exchangers are assumed to scavenge metal ions via cation exchange.
Vinyl triazine compounds include vinyl triazine and derivatives thereof or their acid adducts, among which preferred are 2,4-diamino-6-vinyl-S-triazine represented by formula (1), 2,4-diamino-6-vinyl-s-triazine-isocyanuric acid adduct represented by formula (2) and 2,4-diamino-6-methacryloyloxyethyl-s-triazine-isocyanuric acid adduct represented by formula (3). These triazine compounds are assumed to scavenge dissociating metal ions via complex formation. 
Those metal ion scavengers having a smaller particle diameter than that of conductive particles are suitable. Such metal ion scavengers preferably have an average particle diameter of 0.1 xcexcm or more but less than 10 xcexcm, more preferably 0.1 xcexcm or more but less than 3 xcexcm. Electric connectability during connection can be improved by using a metal ion scavenger having an average particle diameter smaller than that of conductive particles.
Metal ion scavengers preferably have a specific surface area of 0.8 m2/g or more but 100 m2/g or less, preferably 1 m2/g or more but 50 m2/g or less, whereby they have many opportunities to come into contact with metal ions to effectively prevent migration.
In addition to the above components, thermoplastic resins may also be incorporated to confer applicability or film-forming properties on bonding materials of the present invention. These thermoplastic resins include phenoxy resins, polyester resins, acrylic resins or the like.
Bonding materials of the present invention may also contain other additives, if desired, such as silane-coupling agents, surfactants, antiaging agents or the like for improving affinity for glass substrates.
The ratio of thermoplastic resins to thermosetting resins may be 0% by weight or more but 100% by weight or less, preferably 0% by weight or more but 99% by weight or less. Metal ion scavengers are incorporated at 1% by weight or more but 60% by weight or less, preferably 3% by weight or more but 50% by weight or less on the basis of the total amount of these resin components. Other additives may be incorporated at 10% by weight or less, preferably 5% by weight or less of bonding materials. Conductive particles may be added at 1 vol % or more but 50 vol % or less, preferably 1 vol % or more but 30 vol % or less on the basis of bonding materials comprising the above components.
Bonding materials of the present invention can be in the form of a paste- or film-like product. Paste-like products may be solvent-free depending on the choice of materials, but normally prepared by dissolving or dispersing the above components in a solvent such as alcohols, ketones, esters, ethers, phenols, acetals, nitrogen-containing hydrocarbons, specifically toluene, methyl ethyl ketone, ethyl acetate, cellosolve acetate, etc. Solvents are used in an amount of about 20% by weight through 40% by weight of resin components.
Film-like products can be formed by applying the above paste in the form of a film on a release sheet such as release-treated polyethylene terephthalate and evaporating the solvent.
When a bonding material of the present invention is used to connect members to be connected such as plasma display panels or flexible printed wiring boards, the bonding material of the present invention is inserted between these members to be connected and heat and pressure are applied from both sides of the members to be connected, whereby the resin is cured to bond the members. If the bonding material is a paste, the bonding material is applied on a connection region containing electrodes of one member to be connected and, after drying or not, the other member to be connected is superposed and pressed, whereby the paste is cured. When the bonding material is a film, it may be inserted between members to be connected and cured under heat and pressure.
During the above connection process, the bonding material is inserted between members to be connected and heated to melt the resin of the bonding material, whereby the resin of the bonding material flows under pressure from regions containing opposite electrodes to regions containing no electrodes while conductive particles remain between electrodes and come into contact with the electrodes to bond them. The resin component having flown to regions containing no electrodes is cured at these regions with conductive particles dispersed therein to bond the members to be connected. Thus, electric connection between electrodes and mechanical bonding between members to be connected are achieved.
Bonding materials can be cured by not only heating but also irradiation such as UV. In the latter case, bonding materials suitably contain a resin component based on a photopolymerizable resin rather than a thermosetting resin.
Bonding materials of the present invention ensure good mechanical bonding and electric connection even when electrodes have small pitch, area and distance, because they use a metal scavenger having a smaller particle diameter than those of conductive particles.